In recent years, screen printing apparatuses and methods have been used for a cream solder printing process in a circuit mounting process for electronic components. The reduced size and weight of electronics and their improved functions, however, further urge the reduction of the size of electronic-components-mounted circuit boards and the improvement of their accuracy, thereby requiring very accurate printing. Thus, the use of both contact printing and snap-off-speed-controlled printing has become popular.
Contact printing eliminates the gap between a screen and a circuit board in order to minimize the distortion of the screen due to the gap. Snap-off-speed-controlled printing controls the snap-off speed to a low value (in recent years, a common value has been 1 mm/sec.) in order to stabilize printed that is, to minimize the disruption of printed shapes occurring when the screen is pulled due to the viscosity and adhesion of cream solder filled in the opening holes in the screen when the screen is released from the circuit board. Furthermore, in recent years, low-pressure printing is gathering the industry's attention and various attempts are being made to improve this technique. This printing method minimizes the spreading of cream solder to the rear surface of the screen caused by an unnecessarily high printing pressure (a pressure effected when squeegees press the screen) and misalignment caused when the squeegees pull the screen during printing.
An example of low-pressure printing is the screen printing apparatus described in Japanese Patent Application Laid-Open No. 7-205397.
An example of low-pressure printing in a conventional screen printing apparatus and method is generally described below with reference to the drawings.
FIG. 9 shows a schematic front view generally describing a low-pressure printing head section in a general conventional screen printing apparatus and method and also schematically shows a circuit diagram of part of a pneumatic circuit in the low-pressure printing head section.
In FIG. 9, 1 is a squeegee, 2 is a squeegee holder for gripping and holding the squeegee 1, 3 is a holder for holding the squeegee holder 2, 4 is an air cylinder for both rods that elevatorily drives the squeegee 1, 5 is an elevating guide used to elevatorily drive the squeegee 1, 6 is a slide bearing for the elevating guide 5, 7 is an upper piston rod of the air cylinder 4, 8 is a stopper for adjusting the elevating stroke of the air cylinder 4, 9 is a lower piston rod of the air cylinder 4, 10 is a base block secured to the tip of the lower piston rod 9, and 11 is a rotation-supporting shaft secured to the base block 10.
The holder 3 is fitted on the base block 10 in such a manner that it can be rotationally oscillated using the rotation-supporting shaft 11.
Reference numeral 12 designates a micrometer head secured to the base block 10 wherein the amount of the protrusion of its tip can be adjusted so that the tip abuts the holder 3. Reference numeral 13 denotes a spring mounted between the base block 10 and the holder 3, and the micrometer head 12 and the spring 13 are located at approximately symmetrical positions about the rotation-supporting shaft 11. Reference numeral 14 indicates a clamp lever that fixes the rotational oscillation of the holder 3.
Reference numeral 15 designates a manual regulator that transmits to a lower port 16 in the air cylinder 4 the pressure of air passing the regulator 15 when the squeegee 1 is elevated. The air pressure must be manually adjusted in advance using the manual regulator 15 so as to account for the weight of the squeegee 1 and associated components operating in response to the squeegee 1 as well as the resistance effected during elevation and lowering.
Reference numeral 17 denotes an electropneumatic regulator that transmits to an upper port 19 in the air cylinder 4, a total air pressure including the minimum force required to push down the squeegee 1 elevated due to the air pressure adjusted by the manual regulator 15 and an additional air pressure required to push the squeegee down to the screen 18, when the squeegee 1 is lowered.
The parallelism between the screen 18 and the squeegee 1 is visually adjusted using the micrometer head 12.
This configuration enables printing on the screen 18 and the circuit board 20 at a low pressure at which the force equal to the weight of the squeegee 1 and associated components operating in response to the squeegee 1 cancels the force for pressing the squeegee onto the screen 18.
The above configuration, however, has the following five problems, so it cannot meet the request for accurate printing that can be achieved by improving the operability, setting accuracy, and reproducibility during the setting of printing conditions.
(Problem 1)
Since the micrometer head 12 abuts the supported portion of the squeegee 1, printing fails to flexibly fit the slope, swell, and warp of the circuit board 20 in the printing direction or the direction perpendicular thereto, resulting in unbalanced and thus inaccurate printing (printed shapes and volume).
(Problem 2)
The variation of squeegees 1 occurring during manufacturing causes misalignment between the center of the squeegee 1 and its center of gravity to cause the squeegee 1 to be tilted to one side around the rotation-supporting shaft 11, resulting in different printing pressures around the rotation-supporting shaft 11.
(Problem 3)
A squeegee 1 the length of which corresponds to the size of the circuit board is used at the production site, and the weight of the squeegee depends on its length or material, so the air pressure from the manual regulator 15 must be adjusted as required. Likewise, the air pressure from the electropneumatic regulator 17 must be changed to re-adjust the total pressure.
Furthermore, since the weight of the squeegee 1 is unknown, adjustments are required based on the results of eye estimations or measurements conducted by a separate measuring device. Consequently, unwanted arrangement time is required and the operability, setting accuracy, and reproducibility are degraded.
To alleviate this problem, the maximum weight of a mountable squeegee 1 can be assumed to adjust the air pressure from the manual regulator 15 in advance in order to eliminate the needs for the operation of adjusting the air pressure from the manual regulator 15 during operation. This, however, cannot obviate the needs for the unwanted arrangements for identifying the weight canceling pressure.
(Problem 4)
Since the parallelism between the screen 18 and the squeegee 1 is visually adjusted using the micrometer head 12, it varies with the operator, thereby degrading the setting accuracy and reproducibility.
A different conventional example (not shown) removes the micrometer head 12 and the spring 13 to provide a rotatable configuration. This configuration obviates the needs for the parallelism adjustments in contrast to the locked configuration. If, however, the center of the printing pattern on the screen 18 is offset from the center of the screen 18, the longitudinal center of the squeegee 1 must be almost aligned with the center of the circuit board 20 in order to improve stability required to maintain the parallelism.
(Problem 5)
When the screen 18 is released from the circuit board 20, printing is executed with the squeegee 1 pressing the screen 18, so the screen 18 may be bent (because the screen 18 is pressed until the air cylinder 4 reaches the end of its lowering stroke) to disrupt printed shapes, thereby degrading the printing accuracy.
It is an object of this invention to solve Problems 1 to 5 shown above.
(Object 1)
It is an object of this invention to achieve balanced printing at a low specified pressure by flexibly fitting the squeegee along the circuit board regardless of its slope, swell, or warp.
(Object 2)
It is an object of this invention to achieve balanced printing at a specified pressure without tilting the squeegee regardless of the position of its center of gravity.
(Object 3)
It is an object of this invention to enable low-pressure printing that cancels the weight of the squeegee in order to minimize the spreading of cream solder to the rear surface of the screen and misalignment caused when the squeegee pulls the screen during printing, and to eliminate an unwanted arrangement operation performed by a skilled operator to determine the weight canceling pressure in order to account for the variation of the weight of the squeegee depending on its length or material, thereby improving the operability, setting accuracy, and reproducibility during the setting of printing conditions.
(Object 4)
It is an object of this invention to automatically control the adjustment of the parallelism between the screen and the squeegee that is otherwise carried out by a skilled operator, thereby improving the operability, setting accuracy, and reproducibility during the setting of printing conditions.
(Object 5)
It is an object of this invention to achieve acceptable printing by minimizing the bending of the screen caused during a snapping-off operation of releasing the screen from the circuit board.